One particularly useful application for conformable electrically conductive compositions is as the interface material between the skin and the electrode plate of a biomedical electrode. For many years conductive materials predominately used in biomedical electrodes were messy gels containing ionizable inorganic salts and humectants to provide the requisite electrical conductivity. Such gels suffer from a number of disadvantages. Because of their high water content, they tend to dry out and lose their conductivity unless elaborately packaged. Also, when the electrode is removed from the skin, the gel leaves a messy residue on the skin requiring clean-up.
Improved alternatives to messy gels and creams have been developed in recent years and the emergence of so-called "dry" electrodes has been the result. Of these dry electrodes, at least two contain a conformable electrically conductive interface material comprising a pressure-sensitive material which has been made electrically conductive by the inclusion of a conductive filler. U.S. Pat. No. 3,911,906 describes such a biomedical electrode in which the preferred pressure-sensitive material is a synthetic acrylic copolymer and the electrically conductive filler is fine carbon powder.
German Patent Application No. DT 2814-061 describes an electrode in which the electrically conductive interface layer is a mixture of carbon fibers and an acrylate adhesive.
U.S. Pat. No. 4,098,945 describes soft conformable electrically conductive compositions comprising a soft polymeric binder such as a pressure-sensitive adhesive, an electrically conductive filler such as carbon black and a plurality of insoluble, nonconductive, deformable, normally tacky and elastomeric microspheres dispersed in the binder. The microspheres impart flexibility and conformability to the compositions while lessening the amount of conductive filler required to provide electrical conductivity. When these compositions were tested for use as the conductive interface material in a bioelectrode, particularly an electrosurgical grounding plate electrode, they were found to provide insufficient electrical conductivity. Increasing the amount of conductive particles in the composition in order to increase conductivity had an adverse effect on physical properties such as softness, conformability and tack.
It was surprisingly discovered that by using a polar polymeric binder in conjunction with non polar microspheres and oleophilic electrically conductive particles, the conductivity of the composition was greatly enhanced without increasing, and in most cases by actually decreasing, the amount of conductive particles required. Although not wishing to be bound by any specific theory, it is believed that the compositions of the present invention exhibit enhanced electrical conductivity by providing better conductive pathways through the binder. The synergistic effect of the polar binder, the nonpolar microspheres and the conductive particles results in a realignment of the conductive particles within the binder matrix. Photomicrographic analysis of the conductive compositions of the invention indicate that the conductive filler is not homogeneously dispersed throughout the binder as is the case with prior art compositions. The conductive particles concentrate around the surface of the microspheres thereby providing better electrically conductive pathways with a minimum amount of conductive filler.